This invention relates to a weighing apparatus using a load cell as its weight sensor and, more particularly, to the technology of correcting the effects of vibrations, for example, from the floor on which such an apparatus is set.
In a weighing apparatus using a load cell as its weight sensor, the load cell has one of its edges fastened to a base and the other end to support the weight of an object to be weighted such that the strain generated therein according to the applied load is converted into an electrical signal by means of gauges. Since the load cell is thus cantilevered, it is easily affected by the vibrations, for example, from the floor on which the apparatus is set. This naturally gives rise to the problem of yielding erroneous results in measurements.
In order to solve this problem, Japanese Patent Publication Tokkai 60-142216 disclosed a technology of providing a weighing apparatus equipped with a second load cell, referred to as the dummy cell, which is structured and cantilevered just like the one for measuring the load and to cancel the vibratory component contained in the weight signal by inputting both the weight signal and the signal from this dummy cell to an operational amplifier. In general, however, a signal outputted from a load cell contains noise components having higher frequencies than mechanical vibrations. Thus, although a low pass filter composed of an analog circuit is frequently used to extract only the low-frequency components in such a situation, the drift of the DC amplifier itself in the low-pass filter and phase shifts due to errors in the circuit constants of capacitors, etc. tend to cause level shifts and phase differences in the signals from the load cells. As a result, the vibrations from the floor cannot be eliminated satisfactorily, causing errors in the measurement.